Gas turbine engines include compressor rotors including a plurality of rotating compressor blades. Minimizing the leakage of air between tips of the compressor blades and a casing of the gas turbine engine increases the efficiency of the gas turbine engine as the leakage of air over the tips of the compressor blades can cause aerodynamic efficiency losses. The abradability of the seal material prevents damage to the blades while the seal material itself wears to generate an optimized mating surface and thus reduce the leakage of air.
Abradable seals have also been used in turbines to reduce the gap between a rotor and a vane. Thermally sprayed abradable seals have been used in gas turbine engines since the late 1960s. The seals have been made as coatings from composite materials that derive their abradability from the use of low shear strength materials or from a porous, friable coating.
The high conductivity of conventional alumina rotor coatings causes thermal runaway events when rub occurs between vanes and the rotor shaft. The runaway event is caused by heat generation during rub that raises the temperature of both the vane tips and the rotor shaft, especially when the rub contact is limited to only a portion of the rotor's circumference. The heat generated causes expansion of the parts, which increases the rub forces, leading to more heat and then more expansion. The cycle becomes self propagating and has resulted in rotor shaft burn through.
In the past, cantilevered vane rubs have been typically limited to less than 2 mils (50.4 microns) and have less than full circumference contact due to the risks of high rub forces, coating spallation or a thermal runaway event where the heat from the rub causes thermal expansion of the rotor. The rotor, when heated sufficiently, can grow out to interfere with the vanes. The result can be a burn through causing holes in the rotating shaft, which can cause subsequent unscheduled engine removal.